Human parainfluenza virus type 3, PIV3, is an important primary cause of severe lower respiratory tract disease in infants and young children. The virus occurs worldwide and infects virtually all children under the age of four. Acute respiratory disease and secondary complications are particularly serious in infants and young children due to the immaturity of the respiratory system and may require hospitalization in severe cases. Lower respiratory infections are referable to all segments of the respiratory tract, are usually associated with fever, cough, runny nose, and fatigue, and are diagnosed clinically as bronchitis, bronchiolitis, pneumonia, croup, or viral infection. Older children and adults are also frequently reinfected although reinfection typically results in less severe upper respiratory tract illness.
Attempts to develop effective PIV3 vaccines have been largely unsuccessful. Clinical studies using live or inactivated PIV3 vaccines demonstrated an increase in virus specific serum antibodies but provided no significant protection against the disease.